Morphologies of z ∼ 0.7 AGN host galaxies in CANDELS: no trend of merger incidence with AGN ...

15 Pages·2014·3.5 MB·English

MNRAS439,3342–3356(2014) doi:10.1093/mnras/stu173
AdvanceAccesspublication2014February27
∼ .
Morphologies of z 0 7 AGN host galaxies in CANDELS: no trend
of merger incidence with AGN luminosity
C. Villforth,1,2‹ F. Hamann,1 D. J. Rosario,3 P. Santini,4 E. J. McGrath,5
A. van der Wel,6 Y. Y. Chang,6 Y. Guo,7 T. Dahlen,8 E. F. Bell,9 C. J. Conselice,10
D. Croton,11 A. Dekel,12 S. M. Faber,7 N. Grogin,8 T. Hamilton,13 P. F. Hopkins,14,15
S. Juneau,16 J. Kartaltepe,17 D. Kocevski,18 A. Koekemoer,8 D. C. Koo,7 J. Lotz,8
D. McIntosh,19 M. Mozena,7 R. Somerville20 and V. Wild2
1DepartmentofAstronomy,UniversityofFlorida,Gainesville,FL32611,USA
2SUPA,SchoolofPhysicsandAstronomy,UniversityofSt.Andrews,NorthHaugh,St.Andrews,FifeKY169SS,UK
D
3Max-Planck-Institutfu¨rExtraterrestrischePhysik(MPE),Postfach1312,D-85741Garching,Germany ow
4INAF–OsservatorioAstronomicodiRoma,viadiFrascati33,I-00040MontePorzioCatone,Italy nlo
5DepartmentofPhysicsandAstronomy,ColbyCollege,Waterville,ME04901,USA ad
6MaxPlanckInstituteforAstronomy,Ko¨nigstuhl17,D-69117Heidelberg,Germany ed
7UCObservatories/LickObservatoryandDepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064,USA fro
m
8SpaceTelescopeScienceInstitute,3700SanMartinDr.,Baltimore,MD21218,USA h
9DepartmentofAstronomy,UniversityofMichigan,AnnArbor,MI48104,USA ttp
10SchoolofPhysicsandAstronomy,UniversityofNottingham,NottinghamNG72RD,UK ://m
11CentreforAstrophysicsandSupercomputing,SwinburneUniversityofTechnology,POBox218,Hawthorn,VIC3122,Australia n
12RacahInstituteofPhysics,TheHebrewUniversity,Jerusalem91904,Israel ras.o
13DepartmentofNaturalSciences,ShawneeStateUniversity,Portsmouth,OH45662,USA xfo
14TAPIR,Mailcode350-17,CaliforniaInstituteofTechnology,Pasadena,CA91125,USA rd
15DepartmentofAstronomyandTheoreticalAstrophysicsCenter,UniversityofCaliforniaBerkeley,Berkeley,CA94720,USA jou
16CEASaclay,IRFU/SAp,F-91191Gif-Sur-Yvette,France rna
17NationalOpticalAstronomyObservatory,950N.CherryAve.,Tucson,AZ85719,USA ls.o
18DepartmentofPhysicsandAstronomy,UniversityofKentucky,Lexington,KY40506,USA rg
1290DDeeppaarrttmmeennttooffPPhhyyssiiccssaannddAAssttrroonnoommyy,,URuntigveerrssiUtynoivfeMrsiistsyo,u1r3i6-KFarnesliansgChuityys,e5n1R10oaRdo,cPkihsicllaRtaowaady,,KNaJn0sa8s85C4i,tyU,MSAO64110,USA at Ca/
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Accepted2014January20.Received2014January17;inoriginalform2013October28 ia
In
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titu
te
ABSTRACT o
The processes that trigger active galactic nuclei (AGN) remain poorly understood. While f Te
c
lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger hn
o
disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies lo
g
y
areidealenvironments forAGNtriggeringsincethey provide largegas supplies and galaxy o
n
scaletorques.Thereishoweverlittleobservationalevidenceforastrongconnectionbetween M
a
y
AGNandmajormergers.WeanalysethemorphologicalpropertiesofAGNhostgalaxiesasa 2
9
functionofAGNandhostgalaxyluminosityandcomparethemtoacarefullymatchedsample , 2
0
of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z <0.8 and have 14
luminosities41(cid:2)log(L [ergs−1])(cid:2)44.5.‘FakeAGN’aresimulatedinthecontrolgalaxies
X
byaddingpointsourceswiththemagnitudeofthematchedAGN.WefindthatAGNhostand
control galaxies have comparable asymmetries, Se´rsic indices and ellipticities at rest frame
∼950nm.AGNhostgalaxiesshowneitherhigheraverageasymmetriesnorhigherfractions
of very disturbed objects. There is no increase in the prevalence of merger signatures with
AGN luminosity. At 95percent confidence we find that major mergers are responsible for
<6percentofallAGNinoursampleaswellas<40percentofthehighestluminosityAGN
(log (L [erg s−1]) ∼ 43.5). Major mergers therefore either play only a very minor role in
X
(cid:2)
E-mail:[email protected]
(cid:3)C 2014TheAuthors
PublishedbyOxfordUniversityPressonbehalfoftheRoyalAstronomicalSociety
CANDELS:hostgalaxiesofz ∼0.7AGN 3343
thetriggeringofAGNintheluminosityrangestudiedortimedelaysaretoolongformerger
featurestoremainvisible.
Key words: galaxies: active–galaxies: evolution–galaxies: interactions–galaxies: irregu-
lar–quasars:general.
nitudesinsomebands(McLure&Dunlop2002;Marconi&Hunt
1 INTRODUCTION
2003)]. Theoretical models posit that major mergers of gas-rich
Supermassiveblackholes(SMBHs)arenowbelievedtobepresent galaxies trigger both starbursts and AGN, and the AGN subse-
in the centres of most if not all massive galaxies (e.g. Kormendy quently shuts down the star formation by depositing energy into
& Ho2013, and references therein). While most SMBHs do not the interstellar medium (ISM; commonly termed AGN feedback)
accretelargeamountsofgas,asmallfractionofthemshowstrong andtherebyestablishestheM–σ relation(Silk&Rees1998;King
signsofaccretion.Theseobjectsareknownasactivegalacticnuclei 2003;DiMatteo,Springel&Hernquist2005;Springel,DiMatteo&
(AGN).TheconditionsunderwhichSMBHsbecomeactiveremain Hernquist2005;Hopkinsetal.2008;Somervilleetal.2008).Al- D
o
poorly understood. AGN activity requires (a) the availability of though some authors have pointed out that repeated mergers of w
n
eithergasorstarstofeedtheblackholeand(b)aprocesstostrip galaxies containing black hole seeds explain the correlations as lo
a
saidmaterialofitsangularmomentum.Depositinglargeamounts well (Peng 2007; Jahnke & Maccio 2011; Angle´s-Alca´zar et al. de
d
ofgasinthecentresofgalaxiesmakesAGNactivityprobablesince 2013). fro
it provides material, as well as an ideal environment to transfer Despiteitshightheoreticalappeal,observationalevidencefora m
h
angularmomentum. connectionbetweenmergersandAGNremainsmixed.Forcertain ttp
Differentprocessescouldprovidesuchafavourableenvironment samplesofAGN,ratesofrecentmergersareextremelyhigh.This ://m
forAGNtriggering:majorandminormergersofgalaxies(e.g.Silk is true for local quasars that also show large far-infrared (FIR) n
ra
& Rees 1998; Springel, Di Matteo & Hernquist 2005 ; Di Matteo luminosities–similartoultraluminousinfraredgalaxies(ULIRGs) s.o
x
et al. 2008; Hopkins et al. 2008), bars (e.g. Shlosman, Frank & –aswellaspeculiarAGNsamplessuchasredquasars(Canalizo& fo
Begelman 1989), close passages of galaxies disturbing the gravi- Stockton2001;Urrutia,Lacy&Becker2008).However,whenhost rd
jo
tational potential (e.g. Hopkins et al. 2008), cooling of gas from galaxiesofAGNarecomparedtogalaxiesofsimilarmasstheyshow urn
thehothalo(e.g.Crotonetal.2006;Fabian2012;Pope,Mendel& comparableincidencesofdisturbancesindicativeofrecentmergers als
Shabala2012),masslossfromstellarwinds(e.g.Daviesetal.2012), (Dunlopetal.2003;Groginetal.2005;Pierceetal.2007,2010; .org
coldflowsincombinationwithviolentdiscinstabilities(Dekeletal. Gaboretal.2009;Cisternasetal.2011;Kocevskietal.2012;Boehm a/
2009;Bournaudetal.2011),galaxyscaletorques(Angle´s-Alca´zar, etal.2013).However,hostgalaxiesofmoderatelyluminousAGN t C
a
O¨zel&Dave´ 2013)aswellasaccretionofsmallamountsofgas and radio-selected AGN show weak merger features with higher lifo
fromthehalo(King&Pringle2007). surfacebrightnessesthanfoundincontrolgalaxies(Bennertetal. rn
ia
AGN of different luminosities require vastly different amounts 2008;RamosAlmeidaetal.2012).Treisteretal.(2012)studiedthe In
ofaccretionmaterial.GiventypicalAGNlifetimesof108 yr(see incidenceofmergerfeaturesasafunctionofAGNluminosityand stitu
e.g.Martini&Weinberg2001;Yu&Tremaine2002;Martini2004, foundthatthehighestluminosityAGNhavehigherincidencesof te
o
and references therein, although a wide range of AGN lifetimes mergers.However,thesamplewasnotuniformlyselectedandno f T
is possible) a low-luminosity AGN of log(Lbol [erg s−1 ]) = 42 control samples were used. It is therefore unclear if these results ech
requires as little as 2 × 104M(cid:4), while a luminous AGN of willholdinwell-controlledstudies. no
log(Lbol [erg s−1]) = 46 requires as much as 2 × 108M(cid:4). This Inthisstudy,weexaminetheincidenceofdisturbancesindicative logy
correspondstoabout1percentofthetotalgasmassintypicalmas- of major mergers in AGN hosts as a function of AGN luminos- o
n
sivegalaxies(Catinellaetal.2010;Saintongeetal.2011).Stripping itycomparedtoacarefullymatchedcontrolsampleusingCosmic M
a
suchasubstantialfractionofthegasmassinagalaxyoflargeparts AssemblyNear-infraredDeepExtragalacticLegacySurvey(CAN- y
2
ofitsangularmomentumintheshortlifetimeoftheAGNischal- DELS)(Groginetal.2011;Koekemoeretal.2011)datainGreat 9, 2
lenging.Itishenceexpectedthatwhileawiderangeofprocesses Observatories Origins Deep Survey-South (GOODS-S). One par- 0
1
4
canleadtothetriggeringoflow-luminosityAGN,high-luminosity ticular goal of this study is to test the hypothesis that mergers
AGNrequiresubstantialdisturbancestotheirhostgalaxies.Major dominatetriggeringinmoreluminousAGN.Weselectasampleof
mergers of galaxies are therefore thought to dominate triggering AGNspanningawiderangeinluminositiesusingtheirX-rayemis-
atthehighestAGNluminosities.Whilethereisnoclearpredicted siononly.ThemostluminousAGNinoursamplerequireaccret-
breakpoint,atransitionislikelyaroundlog(L [ergs−1])=46,as ing material around 108M(cid:4) assuming standard quasar lifetimes.
bol
arguedabovealthoughsomeauthorssuggestthatatlowerredshift, We aim to answer the question of whether merger triggering be-
majormergersdonotdominateoverthefullAGNluminosityrange comesmoreimportantwithincreasingAGNluminosityand,ifso,
(Draper&Ballantyne2012,seealsoHopkins,Kocevski&Bundy atwhatAGNluminositymergersbecomethedominantmechanism.
2013). Theincidenceofmajormergerswillbeassessedusingquantitative
ThetopicofAGNtriggeringbecamerelevantforgalaxyevolu- morphological measures that determine the level of disturbance
tionwhenitwasdiscoveredthatSMBHsarenotonlycommonin in the host galaxy (Conselice, Bershady & Jangren 2000). While
massivegalaxiesbuttheirmassesalsocorrelatewellwiththeprop- manystudiesrelyonhumanclassifiers(e.g.Cisternasetal.2011;
ertiesoftheirhostgalaxies[velocitydispersion(e.g.Ferrarese& Kocevskietal.2012;Treisteretal.2012),usingquantitativemea-
Merritt2000;Gebhardtetal.2000;Tremaineetal.2002;Gu¨ltekin suresenablesustodetectmoresubtlelevelsofdisturbance(Con-
etal.2009,2011),stellarmass(e.g.Ha¨ring&Rix2004),central seliceetal.2000;Lotzetal.2010a,b)aswellasallowmoredetailed
lightconcentration(Grahametal.2001)aswellasabsolutemag- statisticalanalysisoftheresults.
MNRAS439,3342–3356(2014)
3344 C.Villforthetal.
The paper is organized as follows. Data reduction and analy- Fromthe4MsCDF-Ssample,westudyall76objectscovered
sis as well as sample selection are presented in Section 2. The in CANDELS in the redshift range 0.5 < z <0.8. Additionally,
results are presented in Section 3, followed by discussion in we reject objects with soft X-ray spectra ((cid:6) > 1, where (cid:6) is the
Section4andsummaryandconclusioninSection5.Supplemen- effective photon index) that lack a point source detection in the
tal information about morphological measures used and simula- F160W data since these sources are likely starbursts. We caution
tionsperformedarepresentedinAppendixA.Thecosmologyused that extreme Compton thick sources can be potentially rejected
isH0=70kms−1Mpc−1, (cid:4)(cid:5)=0.7 , (cid:4)m=0.3.Throughoutthe using this method and this method is therefore not 100percent
paper,weuseABmagnitudes. effectiveinidentifyingstarbursts(Juneauetal.2011).Therejection
affectssourceswithdetectionsinonlyasingleChandrabandand
verylowluminosities(seeFig.1),leavingasampleof60AGNin
2 DATA AND ANALYSIS
the field. The X-ray luminosity distributions of the AGN as well
Forthestudy,weusetheF160W/HbandimagingdatainGOODS- astherejectedstarburstsourcesareshowninFig.1(right).Note
South(Groginetal.2011).Thedatareductionisdescribedindetail that the redshift range chosen for this study includes a cluster of
in Koekemoer et al. (2011). Throughout the paper, when citing galaxiesataredshiftof∼0.75(Salimbenietal.2009;Castellano
magnitudes, we refer to the observed frame F160W band, which etal.2011).TheclusterhasaM200∼3×1014M(cid:4)andavelocity
correspondstoarest-framewavelengthof∼950nmintheredshift dispersionofσ ∼630kms−1. D
o
rangestudied. w
n
lo
a
2.2 Matchedsample d
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2.1 AGNsample Forthecontrolsample,weusecataloguesbyDahlenetal.(2010), d fro
m
ThesampleisselectedfromtheChandraDeepField-South(CDF- includingphotometricdataoverawiderangeofwavelengths(see h
S)4Msdata(Xueetal.2011).X-rayselectionprovidesaminimally Giavaliscoetal.2004;Dahlenetal.2010)aswellasacompilation ttp
biasedsampleoverawiderangeofluminosities.Itisleastaffected of spectroscopic redshifts (Cristiani et al. 2000; Croom, Warren ://m
n
byobscurationandatthegreatsensitivityofthedataused,complete &Glazebrook2001;Dickinsonetal.2004;LeFe´vreetal.2004; ra
s
downtolowAGNluminosities.ItshouldhoweverbenotedthatX- Stanwayetal.2004;Strolgeretal.2004;Szokolyetal.2004;van .o
x
rayselection,evenwiththegreatdepthofthe4Msdatawillmissthe derWeletal.2004;Dohertyetal.2005;Mignolietal.2005;Roche fo
rd
mostCompton-thickAGN.However,thesesystemsareexpectedto etal.2006;Ravikumaretal.2007;Vanzellaetal.2008;Popesso jo
u
berare(seee.g.Juneauetal.2011,andreferencestherein).Fig.1 etal.2009). rn
a
(left)showsthefull4Mssampleaswellasthewavelengthrange TheAGNhostgalaxysampleisdominatedbymassivegalaxies, ls
.o
chosenforthisstudy.Thechosenredshiftrangecoversamaximum whilethefullgalaxysampleinthesameredshiftrangegenerally rg
amountofdynamicalrangeinX-rayluminositywhilenotcovering containsmanymorelowermassgalaxies.Thisismostlyaneffect at C/
toolargearedshiftrangetohavesignificantcosmologicalevolution ofdetectionprobabilitysincelow-massgalaxieshavelowermass a
withinthesample((cid:2)2Gyrincosmictime).Surfacebrightnessdim- black holes which makes detection at equal Eddington rate less lifo
rn
mingeffectsareminimal.Throughoutthepaper,weuseabsorption likely(Airdetal.2012).ThehistogramsofAGNhostgalaxiesand ia
corrected rest-frame 0.5–8 keV luminosities in erg s−1 from Xue the parent control sample are shown in Fig. 2. From this parent In
s
et al. (2011). X-ray sources are matched to the H band using a controlsample,wecreateacontrolsamplebymatchingbetween5 titu
1-arcsecaperture. and25galaxiestoeachAGN(Fig.4). te o
f T
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c
h
n
o
lo
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y
o
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M
a
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2
9
, 2
0
1
4
Figure1. Basicpropertiesofthesample.Left-handpanel:absorptioncorrectedX-rayandbolometricluminositiesofX-raysourcesintheCDF-S4Ms
catalogue(greydots),thesampleusedinthispaper(redcircles).Right-handpanel:histogramsandkerneldensityestimators(KDE)ofbolometricluminosities
intheredshiftrangeforallX-ray-sourcesused,objectsrejectedasstarburstsareshownascross-hatched.Inbothpanels,thedottedredlinesshowtheredshift
rangeused,thedashedredlineshowsabolometricluminosityoflog(Lbol [ergs−1])=45.5.Thisistheluminosityabovewhichmergersarethoughtto
dominateAGNtriggering(Somerville&Primack1999;Somervilleetal.2008;Hopkinsetal.2013).
MNRAS439,3342–3356(2014)
CANDELS:hostgalaxiesofz ∼0.7AGN 3345
D
o
w
n
lo
Figure4. MatchingperformedforallAGN,theblackdotsshowthefull ad
e
F(riegdu)raes2w. eHllisatsogarlalmgsalsahxoiewsiningtthheedreisdtsrihbiufttiroannsgoefotfhethAeGsNtudhyos(thgaatclahxeide)s. cthoanttrhoalvseambepelne.mTahtechreeddsatsarcsomntarrokltghaelaAxGieNs,,tthheeggrreeeennldinoetssschoonwnegcatlathxeiems d fro
m
Notethatsincethehistogramsarebothnormalizedtointegratetoone,this totheAGNtheyhavebeenmatchedto.Notetheclusterataredshiftof h
histogramdoesnotrepresentthetotalnumberofgalaxiesinbothsamples. z∼0.75. ttp
://m
galaxies are amongst the most luminous and massive galaxies in n
ra
the field and redshift range (see Fig. 4). These AGN hosts are s.o
matchednon-isotropicallytocontrolgalaxiesoflowermass.While xfo
thisisnotoptimal,wesimplylackappropriateoptimallymatched rd
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galaxiesfortheseAGN.However,wefindthatthemorphological u
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properties(inparticularasymmetryandellipticity)donotshowa als
strongtrendwithabsolutegalaxymagnitude.Additionally,alarge .org
percentageofobjectsarecloselymatched.Ourresultsholdwhen a/
omitting the two AGN with the most anisotropic matching. The t C
a
AGNwithanisotropicmatchingdonotbelongtothebinwiththe lifo
mostluminousAGN. rn
ia
Objectsforwhichstellarmassesarenotavailablearematchedin In
s
Hbandgalaxymagnitudeinstead(seeSection2.3foradescription titu
offitsdescribedtoderiveAGNhostmagnitudes).ForAGNlocated te
in the cluster at z ∼ 0.75, we reject all control galaxies that are of T
outside the clusters redshift range. Since the cluster environment ec
h
can have a strong influence on galaxy morphological parameters n
o
(Dressler1980;Adamsetal.2012),matchingAGNhoststocontrol log
y
Figure3. ComparisonbetweenstellarmassesandabsoluteH-bandmagni- galaxies located in very different environments could introduce o
n
tudeforfullcontrolsample(greydots,allgalaxiesingivenredshiftrange) biases.Theresultsofourstudyholdwhenmatchingisperformed M
andAGNhostgalaxies(redfilledcircles). purelyinH-bandmagnitude. ay
2
9
, 2
Controlgalaxies are matched in absolute F160W(host) galaxy 0
2.3 Hostgalaxyfits 14
magnitude and stellar mass from Santini et al. (2012) as well as
redshift.Thestellarmassesarederivedbyfittingthespectralenergy GalaxyfitsareperformedusingGALFIT(Pengetal.2002).Empir-
distributions (SEDs) of the AGN with a mixture of galaxy and icalpointspreadfunctions(PSFs)werederivedfromstackingthe
quasi-stellarobject(QSO)templates,foramoredetaileddescription imagesofseveralisolatedandunsaturatedstarsinthefield.Inorder
oftheprocess,wereferthereadertoSantinietal.(2009,2012).The to provide a more accurate description of the central region, we
stellarmassistracedratherwellbytheH-bandmagnitude(Fig.3) replaced the innermost pixels (within a radius of 3 pixels from
withascatterof∼0.25dexinstellarmass.Someofthisscatterin thecentre)withasimulatedPSFgeneratedwiththeTINYTIMpack-
stellarmassisexplainedbydifferencesinstarformationhistories age(Krist1995).TheTINYTIMPSFwasditheredanddrizzledinthe
(i.e.starburstage,reddening). samemannerastheobservations,andnormalizedsuchthatthetotal
Initial matching is performed by selecting galaxies with fluxofthenewlyconstructedhybridPSFmodelisthesameasthat
(cid:7)z = 0.05 and (cid:7)m = 0.1, stellar masses are matched within ofthestackedstar.WefoundthishybridPSFaccuratelyreproduced
10percent.Ifnosufficientnumbersofmatchesarefound,thecrite- thegrowthcurvesofstarsoutto3arcsec.FurtherdetailsonthePSF
riaarerelaxed.AscanbeseeninFig.4,mostgalaxiesarematched modelscanbefoundinvanderWeletal.(2012).
isotropically in redshift and magnitude. At lower galaxy magni- For the AGN host galaxies, we use a mixture of point source
tudes,AGNhostscanbematchedtocontrolgalaxieswithrelatively and Se´rsic component (with both ellipticity and radius left as a
largemagnitudedifferences(∼0.5(cid:7)mag).Additionally,twohost freeparameter).Ifnecessary,asecondSe´rsiccomponentisadded.
MNRAS439,3342–3356(2014)
3346 C.Villforthetal.
Inmostcases,afitwithapointsourceandsingleSe´rsicyieldeda fits. Relaxed post-merger systems could show lower ellipticities
goodfitwithminimalresiduals.Thegoodnessoffitwasjudgedboth than galaxies of similar mass that have not undergone a merger
basedontheχ2valuesgivenbyGALFITandvisualinspectionsofthe recently. A detailed description of the morphology measures and
residuals.ItwasonlynecessarytoaddasecondSe´rsiccomponent possibleerrorsduetoPSFresidualscanbefoundinAppendixA.
in four cases out of the 60 AGN studied. In these cases, the fits Hereweprovideabriefoverview.
divergedforasingleSe´rsiccomponentorresultedinstrongresiduals AsymmetryAisdefinedas
(cid:2)
indicativeofthepresenceofacomponentnotaccountedforinthe (cid:3)
fits.Sinceweusethepoint-sourcesubtractedimagesratherthanthe A≡ 12((cid:3)I0−I180)2, (1)
residualimagesforfurtheranalysis,detailsinthehostgalaxyfits I2
0
themselvesdonotstronglyaffectourresults.
whereI isthefluxineachpixelandI isthefluxineachpixel
0 180
Theimageresolutionof0.1arcseccorrespondstoabout0.5kpc rotatedby180◦ (Conselice2003).DifferentfromConseliceetal.
intheredshiftrangeofoursample.Acompactbulgeorstarburst
(2000)andConselice(2003),wedonotsubtractbackgroundasym-
mightthereforebefitasapointsource.ForallAGNthatrequire
metry.However,asshowninConseliceetal.(2000),thiswillhave
pointsourcefits,wethereforecheckthecolourofthecentralpoint
littleeffectforthetypicallybrightgalaxiesusedinthisstudy.For
sourceusingAdvancedCameraforSurveys(ACS)images.Colours
thepurposeofthisstudy,weuseSEXTRACTORsegmentationmapsto
ofcentralpointsourcesareblue,consistentwithAGN.Starbursts avoidincludingnoisefromthebackgroundintothemeasurement. Do
however have similar colours. We find that contributions of cen- w
Thesemapsdeterminetheregionoverwhichthegalaxyisdetected. n
tral point sources in low-luminosity AGN are weak. There is the Inthefollowing,wewilldiscussdifferentinfluencesonthemea- loa
possibilitythatAGNcontributionsareoverestimatedatlowX-ray suredasymmetry.CentringisperformedfollowingConseliceetal. ded
luminosity, however, our results hold when omitting AGN with (2000). We have ensured that the algorithm generally reaches a fro
log(L <42)andthereforethisdoesnotaffecttheoverallconclu- m
X well-definedminimum.VisualinspectionofallAGNhostsanda h
sionsofthisstudy. randomlychosensubsetofthecontrolsampleisperformedtomake ttp
AllControlgalaxiesarefitwithasingleSe´rsiccomponentandno surethecentralpointdeterminedbythealgorithmdeterminesthe ://m
pointsourcetodeterminetheirSe´rsicindicesaswellasellipticities. n
centreofthegalaxycorrectly.Threegalaxieswithdifferentlevels ra
To mimic residuals from PSF fitting performed for AGN hosts, ofasymmetryareshowninFig.5asareferenceforthereader. s.o
x
wecreatefakeAGNforthemorphologicalanalysis.Pointsources CentralpixelsareinsomecasesaffectedbyPSFresiduals.While fo
withmagnitudesmatchedtothecorrespondingAGNmagnitudeare rd
simulating ‘fake AGN’ ensures that this is also the case for the jo
addedtoitsmatchedcontrolgalaxies.The‘fakeAGN’arethenfit control sample, we still do not wish these pixels to dominate the urn
wofitthheaipnofliunetnscoeusrcoefarensdidSue´arlssiconmmodoerlp.hAolomgoicrealdpeatarailmedetdeirssccuasnsiobne ojevcetrsailslamsyasmkmedetursyinmgeaascuirrecmuleanrta.pHeerntucree,wthiethceanrtaradliuasreoafo2fpalilxeolbs-. als.org
foundinAppendixA. Theexactsizeofthemaskdoesnotchangetheoverallresultand at C/
visualinspectionsshowthatfortheAGNmagnitudescommonin a
our sample, these aperture sizes cover the corrupted pixels while lifo
2.4 Quantitativemorphologymeasures:asymmetry rn
notmaskinguncontaminatedareasofthehostgalaxy.Moredetails ia
Forquantitativemorphologymeasurements,weuseasymmetryA canbefoundinAppendixA. Ins
which is part of the compactness–asymmetry–smoothness (CAS) Inordertokeepasymmetrymeasurescomparableindependentof titu
classification system (Conselice 2003). Tests on our data showed opticalAGNmagnitude,weusethesamemasksizeforallobjects, te o
thattheconcentrationCwasmostsensitivetoPSFresidualssince independentofAGNmagnitude.Whilethisalsomasksthecentral f T
e
itworksbymeasuringhowcentrallyconcentratedgalaxiesarein areasofgalaxiesnotaffectedbyPSFresiduals,itensuresthatthe ch
n
agreementwithpreviousstudies(Groginetal.2005).Becauseofthe samephysicalareasareusedforthecalculationoftheasymmetry olo
limitedresolutionofourdata,smoothnessSisnotused.Asymmetry forallsources.Ideally,themasksizewouldbeadjustedtothesize gy
o
isfoundtotracemajorandminormergerswell(Lotzetal.2010a,b). ofthegalaxy.However,sinceAGNhostsandcontrolarecarefully n
M
Additionally,wewillusetheellipticityfromtheGALFIThostgalaxy matched,thisshouldnotresultinabiasedmeasurement. a
y
2
9
, 2
0
1
4
Figure5. ThreeexampleimagesshowingdifferentlevelsofasymmetryinthreeAGNhostgalaxies.Allimagesareonalogarithmicscaleand4.2×4.2arcsec2
insize.
MNRAS439,3342–3356(2014)
CANDELS:hostgalaxiesofz ∼0.7AGN 3347
2.5 Humanclassifiers thesampleoftheircontrolgalaxies.NotethatwhenplottingX-ray
luminosity,weplottheX-rayluminosityofthematchedAGNfor
We use results from human classifiers for comparison and con-
thecontrolgalaxies.
sistencychecks.ThegalaxieswereclassifiedbyCANDELSteam
WecomparethedistributionofSe´rsicindicesfortheAGNand
members in the H/F160W band. Bluer bands are also inspected
controlsample(Fig.6).Twosampletestsshownostatisticallysig-
to facilitate the classification (Kartaltepe et al. 2012, 2014). The
nificantdifferencebetweenAGNhostgalaxiesandthecontrolsam-
boundariesofthegalaxiesaredefinedusedSEXTRACTORsegmasks.
ple.Asexpected,Se´rsicindicesarehigherformoreluminoushost
Inafirststep,theclassifierisaskedtodecidebetweenfourmain
galaxies,thereishowevernostrongtrendwithX-rayluminosity.
morphologicalclasses:spheroid,disc,irregular/peculiarandcom-
Next,weanalysedifferencesbetweentheasymmetriesofAGN
pact/unresolved.Additionally,thehumanclassifierisaskedtode-
host and control galaxies (Fig. 7). The overall distribution of the
cide if the galaxy meets any of the following interaction classes:
asymmetriesforthetwosamplesshowsnosignificantdifferences.
merger(aclearlyinteractingsystemwithdisturbances),interaction
We also find no trend with absolute galaxy magnitude. We per-
(interactionoftwodistinctobjectswithinorbeyondthesegmenta-
formtwo-sampleKolmogorov–Smirnov(KS)andMann–Whitney
tionmask)ornon-interactioncompanion(galaxyhasacompanion
U(MW)testsbetweentheAGNandcontrolgalaxies.Thesetests
thatisnotclearlydisturbed).Thisclassificationschemeisthesame
are calculated for both the full sample and subsamples binned in
asusedinKocevskietal.(2012).
bothX-rayluminosityandabsolutegalaxymagnitude.Theresults D
are shown in Table 1. The two-sample KS test yields p = 0.38 ow
n
2.6 Noteonconfidenceintervalsused andtheMWtestyieldsp=0.09.Wehencefailtorejectthenull loa
hypothesis that the two samples are drawn from the same parent de
d
BinomialprobabilitiesandtheirconfidenceintervalsfortheAGN population.WhenbinnedinX-rayandgalaxyluminosity,theKS fro
samplearederivedusingabetastatistic(Cameron2011).Asstated andMWtestfindnosignificantdifferencesbetweenthesamples. m
inSection2.2,severalcontrolgalaxiesarematchedtoeachAGN Weonlyfindp≤0.05inasinglebin.However,giventhemultiple http
toincreasestatisticalpower.Becauseofthefactthatthenumberof tests(28)performed,thisisconsistentwiththeexpectednumberof ://m
matchesdiffersbetweenAGN,confidenceintervalsforthecontrol falsepositives(1.4). n
sample cannot be derived straightforwardly using the beta statis- Whilethestatisticaltestsrevealnosignificantdifferencebetween ras.o
tics since it does not account for weighting. We therefore use a theasymmetries,wedonotethatbyeyethedistributionofasym- xfo
jackknife method in which we randomly choose a subset of five metriesappearstobeslightlymoreskewedwithalargerhighasym- rdjo
matched control galaxies for each AGN and then derive the re- metrytailintheAGNhostgalaxieswhencomparedtocontrol.To urn
sultingvalueforthebinomialprobability.Thisrandommatchingis determineifthesedifferencesarequantifiable,wecalculatethemo- als
repeated100timesandtheconfidenceintervalsarederivedfromthe mentsofthedistributionsofasymmetriesfortheAGNandcontrol .org
finaldistribution.Jackknifemethodsarealsousedtodeterminethe galaxies.Ajackknifemethodisusedtodeterminethetypicalscatter a/
expectationvaluesanderrorindistributionmomentsforthecontrol inthemomentsderivedforthecontrolsample.Wecalculatethefirst t C
a
sample. fourmoments(mean,standarddeviationσ,skewγ andkurtosisκ). lifo
For the full sample, we find that both the skew and kurtosis are rn
ia
higherfortheAGNsamplecomparedtocontrol.Bothlieabovethe In
3 RESULTS: MORPHOLOGIES OF AGN HOSTS s
thirdquartileofthecontrolsampledistribution.Thisimpliesthat titu
Wenowcomparethemorphologicalproperties(Se´rsicindices,el- thedistributionfortheAGNhostshasatailtowardslargervalues te
o
lipticitiesandasymmetries)oftheAGNhostgalaxiestothoseofthe oftheasymmetryAandahigherpeakwithmorepowerinthetails. f T
controlgalaxies.Theaimistodetermineifthedataareconsistent However,whendividingthesampleintosub-binsineitherAGNor ec
h
withthenullhypothesisthatAGNhostsaredrawnrandomlyfrom hostgalaxyluminosity,thesamplesizesaretoosmalltodetermine no
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Figure6. SersicindexasafunctionofAGNluminosity(left)andAGNhostmagnitude(right)forAGN(redcircles)andcontrol(greydots).Forcontrol
galaxies,theX-rayluminosityofthematchedAGNisplotted.ProjectedhistogramsareshownfortheAGN(red)andcontrolsample(hashed).Thereareno
statisticallysignificantdifferencesbetweenthetwosamples.
MNRAS439,3342–3356(2014)
3348 C.Villforthetal.
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Figure7. AsymmetryasafunctionofX-rayluminosity(left)andAGNhostgalaxymagnitude(right)forAGN(redcircles)andcontrol(greydots).For a
d
controlgalaxies,theX-rayluminosityofthematchedAGNisplotted.ProjectedhistogramsareshownfortheAGN(red)andcontrolsample(hashed).There ed
arenostatisticallysignificantdifferencesbetweenthetwosamples. fro
m
h
Table 1. Results from two-sample KS and MW tests comparing asym- ttp
metriesofAGNhostsandmatchedcontrolsamples.Binproperty:AGN ://m
propertyusedforbinning;Binmean:meanvalueofbinnedproperty;p(KS): nra
p-valueforKolmogorov–Smirnovtests.p(MW):p-valueforMann–Whitney s.o
Utest. xfo
rd
jo
Binproperty Binmean p(KS) p(MW) u
rn
a
log(LX-ray)(all) 42.27 0.34 0.09 ls.o
lllloooogggg((((LLLLXXXX----rrrraaaayyyy)))) 44441112....15938913 0000....81565866 0000....20241770 at Califorg/
log(LX-ray) 43.04 0.10 0.12 rnia
log(LX-ray) 43.60 0.85 0.25 In
s
MHost(all) −22.53 0.38 0.09 titu
MHost −24.02 0.76 0.43 Figure8. PercentiledistributionofAGNasymmetrieswithrespecttotheir te o
MHost −23.10 0.37 0.23 controlsample.ForeachAGN,wecalculatethepercentileatscorewith f T
MHost −22.75 0.85 0.38 respecttoitsmatchedcontrolgalaxies.TheKDEisreflectedoffthebound- ech
MHost −22.32 0.37 0.18 aries.IftheAGNhostasymmetriesweredrawnrandomlyfromthesame no
MHost −21.94 0.18 0.05 distributionasthematchedcontrolgalaxiesasymmetries,thedistribution log
MHost −21.06 0.85 0.43 shouldbeflat(indicatedbythedashedredline).Thedifferenceishowever y o
n
Allcluster – 0.0049 0.0098 oflowstatisticalsignificance(p=0.098). M
a
Allfield – 0.66 0.41 y 2
Obscuredversusunobscured – 0.31 0.17 thefullAGNsampleshouldbeflatsinceeachAGNisequallylikely 9, 2
tobesampledateachpercentileatscore.Weshowthekerneldensity 0
1
4
estimator(KDE)ofthepercentilesatscoreforthefullsamplein
ifthereisatrendinthemoments.Thefullcalculatedpropertiesare Fig.8.Thedistributionisnotflat,showingexcessatlowpercentiles
listedinAppendixB. atscoreaswellasasmallexcessatveryhighpercentilesatscore.
Another way to compare the asymmetries of AGN hosts and This is consistent with the eye ball assessment that the asymme-
controlgalaxiesistocomparethevaluesforeachsingleAGNto triesappearonaveragesomewhatlowerintheAGNcomparedto
itssampleofcontrolgalaxies.Wethereforeanalysethepercentiles control while showing a slight high-asymmetry excess. However,
ofscoresofeachAGNhostgalaxyasymmetrywithrespecttoits duetosmallnumberstatistics,thedifferencesarenotstatistically
matchedcontrolgalaxies.ForeachAGN,wecalculatethecumu- significant(p=0.098).
lative density function of the asymmetries of its control sample. Anadditionalfactorinasymmetrylevelsmightbeobscuration.
Fromthisdistribution,wethencalculatethepercentileatscorefor While it is widely acknowledged that obscuration in AGN in the
eachAGN.Forexample,anAGNwithanasymmetryequaltothe localUniverseismostlyduetoadustytorusandthereforeprimarily
medianasymmetryofitscontrolsamplewillhaveapercentileat afunctionofAGNorientation(Antonucci1993;Urry&Padovani
scoreof50percent,whileanAGNhavingasymmetryhigherthan 1995),someveryyoungAGNmightbeinanearlierburiedphase
all its matched control galaxies will have a percentile at score of in which the obscuration is due to dust in the host galaxy and
100percent.IftheAGNhostsweredrawnrandomlyfromthesam- not the torus. We note that obscuration in the optical and the X-
pleofitsmatchedhosts,thedistributionofpercentilesatscorefor rayisnotnecessarilytracingthesameobscuringmaterial.Sucha
MNRAS439,3342–3356(2014)
CANDELS:hostgalaxiesofz ∼0.7AGN 3349
meanasymmetriesbuthigherskewintheasymmetrydistribution
whencomparedtocontrolgalaxies.
Whileasymmetrytraceslevelsofdisturbanceinthehostgalaxy,
morerelaxedmergerswillnotbeidentifiedbythisindex(forex-
amplemergersbetweenhigh-ellipticitydiscgalaxieswillgenerally
reducetheellipticityinthemergedsystem).Wethusadditionally
comparetheellipticitiesfromtheGALFITgalaxyfitsbetweenAGN
hostsandmatchedcontrols.TheresultsareshowninFig.11.There
arenostatisticallysignificantdifferencesbetweenAGNhostgalax-
iesandcontrolgalaxies.
3.1 Comparisonbetweenquantitativemeasures
andhumanclassifiers
In addition to the distributions of asymmetries, we also compare
theprobabilitiesofobjectshavinghigh(A>0.1)asymmetriesbe- D
tweenAGNhostgalaxiesandcontrolsamples.Thecut-offA>0.1 ow
n
issomewhatarbitrary,butvisualinspectionshowsthistobearea- lo
Figure9. AsymmetryasafunctionofX-rayluminosityforAGN(redand sonablevalueabovewhichallgalaxiesshowcleardisturbance(see ade
bluecircles)andcontrol(greydots).Theplotshowsacomparisonbetween d
X-rayobscured(red)andunobscured(blue)sources.Forcontrolgalaxies, alsoFig.5).Theseratesarecomparedtodifferentmeasuresfrom fro
thehumanclassifierresultsdescribedinSection2.5inFig.12. m
theX-rayluminosityofthematchedAGNisplotted.Projectedhistograms h
areshownfortheobscuredAGN(red),unobscuredAGN(blue)andcontrol Inparticular,wecomparetwodifferentclassifications:irregular- ttp
sample(grey).Therearenostatisticallysignificantdifferencesbetweenthe ityandmerger.Irregularityencompassesallobjectsshowingsome ://m
samples. disturbanceorirregularity,eveniftheyshowawell-pronounceddisc n
ra
orspheroidcomponent.Theclassifiersareaskedtoclassifyobjects s.o
x
subsample of young obscured AGN might be more closely con- asirregulariftheyseeasymmetricfeatures,nottakingintoaccount fo
nectedtomergers.TheasymmetrydistributionsofAGNwithX-ray iftheobjectappearstobeinamergerornot.Thisclassificationis rd
jo
effectivephotonindices(cid:6)<1(X-rayobscured)and(cid:6)>1(X-ray thereforecomparabletotheasymmetryA.Additionally,weusethe urn
unobscured)arecomparedinFig.9.Therearenodifferencesbe- mergerclassification.Classifiersareaskedtoidentifyanyobjects als
tweenthetwoAGNsubsampleasymmetries(Table1),indicating appearingtoundergointeractionorshowinganearbycompanion. .org
thathigherlevelsofobscurationdonotleadtocomparativelylarger Assuch,thiscategoryismorepronetosubjectiveinterpretationsby a/
asymmetries. theclassifiersandisexplicitlynotcomparabletotheasymmetry. t C
a
Asmentionedearlier,theredshiftrangestudiedcontainsacluster Asexpectedfromthesimilaritiesintheoveralldistributions,the lifo
ofgalaxiesatz≈0.75(Salimbenietal.2009;Castellanoetal.2011). ratesofobjectswithhighasymmetries(A>0.1)arenothigherin rn
ia
Weseparatelycomparethehostgalaxiesandtheirmatchedcontrol theAGNhostscomparedtocontrol.Whenstudyingtheratesasa In
s
galaxies in both the cluster and field (Fig. 10). We find that – as functionofX-rayluminosity,nodifferencesbetweentheAGNhosts titu
forthefullsample–theAGNhostsinthefieldareconsistentwith andcontrolarefound.Asafunctionofgalaxymagnitude,thereis te
o
beingdrawnrandomlyfromthecontrolgalaxies.However,inthe amildexcessinhighasymmetryratesatmoderategalaxymasses; f T
cluster,wedofindastatisticallysignificantdifferencebetweenthe withAGNhostshavingslightlyhigherasymmetries.However,this ec
h
AGNandcontrolgalaxies(P<0.01)withAGNhostshavinglower is not statistically significant. Similarly, we find that the rates of no
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Figure10. Asymmetryinthecluster(left)andfield(right)asafunctionofX-rayluminosityforAGN(redcircles)andcontrol(greydots),showingdifferences
betweentheclusterandfield.Forcontrolgalaxies,theX-rayluminosityofthematchedAGNisplotted.ProjectedhistogramsareshownfortheAGN(red)and
controlsample(hashed).Therearenostatisticallysignificantdifferencesbetweenthetwosamplesinthefield,however,inthecluster,thedifferencebetween
AGNhostsandcontrolisstatisticallysignificant(p<0.01).
MNRAS439,3342–3356(2014)
3350 C.Villforthetal.
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Figure11. EllipticityasafunctionofAGNluminosity(left)andAGNhostmagnitude(right)forAGN(redcircles)andcontrol(greydots).Forcontrol loa
d
galaxies,theX-rayluminosityofthematchedAGNisplotted.ProjectedhistogramsareshownfortheAGN(red)andcontrolsample(hashed).Thereareno e
d
statisticallysignificantdifferencesbetweenthetwosamples. fro
m
objectsclassifiedasirregularbyhumanclassifiersshownostatis- Beforecomparingourresultstothoseofotherauthors,wewill http
ticallysignificantdifferencebetweentheAGNhostsandmatched brieflyaddressthesurfacebrightnesslimitanditsinfluenceonthe ://m
control.Wealsofindthatusingacut-offA>0.1leadstosimilar types of merger features detected. We reach surface brightnesses n
overallasymmetry/irregularityratesasclassificationbyhumans. around26magarcsec−2inourstudy.Generally,thebrightestmerger ras
.o
We find the rates of human-classified mergers are significantly features have surface brightnesses as high as 22 mag arcsec−2, xfo
higherintheAGNhostswhencomparedtomatchedcontrol.The while most fainter tidal tails have surface brightnesses down to rd
excessratesforAGNcomparedtocontrolseeninthemergerrates 25 mag arcsec−2 (Elmegreen et al. 2007). Merger features are jou
rn
are due to companions showing no signs of merger – not train- found even in local red and dead elliptical galaxies when depths a
ls
wreckmergersorevenmergersshowingsomesignsofinteraction. of 28 mag arcsec−2 are reached (van Dokkum 2005). However, .o
rg
Asubstantialfractionoftheseobjectsisassociatedwiththecluster attheseextremelyfaintsurfacebrightnesslevels,mergerfeatures a/
environmentatz∼0.75. arealsolikelytotracedisruptionofsmallsatellites(e.g.Johnston, t C
a
Sackett&Bullock2001).Giventhelimitsofourdata,wearethere- lifo
forelikelytodetectongoingmergers,aswellastidaltails,whilethe rn
ia
4 DISCUSSION dataarenotsensitiveenoughtodetectdisruptionofsmallersatel- In
We have compared the morphologies of X-ray-selected AGN to lrietceesn.Htmowajeovrerm,seirngceersw(emaerregperrismwairtihlygianltaexreystmedasisnrtahteioinsc≥id0e.n1celeossf stitute
tihedoseareofamt artecdhsehdiftgazlax=ies0.o5f–0t.h8easnadmehamveassX.-rTahyelAumGiNnossittuides- tohuarnst∼ud1yG.Tyhreinsptahteiaplarsets)o,ltuhtiisonlimofitouisrisdtueadlyfiosr∼th0e.5skcipecn.cWeegomailgohft of Te
c
(log(L [erg s−1]) ≈ 41–44.5). Assuming standard bolometric thereforemissasymmetricfeaturesonverysmallscales. hn
X o
corrections (Nemmen & Brotherton 2010; Runnoe, Brotherton & WewillstartbycomparingourresultstopreviousstudiesofAGN lo
g
y
Shang2012),thebolometricAGNluminositiesreachtovaluesat hostgalaxymorphologiescomparedtocontrolsamplesofquiescent o
n
whichmergertriggeringshouldbecomedominant. galaxies.Anumberofstudieshavefoundnostatisticallysignificant M
ThehostgalaxiesofmoderatelyluminousX-ray-selectedAGN differences(Dunlopetal.2003;Groginetal.2005;Cisternasetal. ay
2
atlowredshiftshownostrongdifferencesinmorphologicalproper- 2011;Boehmetal.2013).Dunlopetal.(2003)studiedsamplesof 9
ties(asymmetries,Se´rsicindices,ellipticities)comparedtogalaxies radio-loud and radio-quiet quasars at z < 0.25 with luminosities , 20
1
matchedinmassorabsoluteH-bandmagnitude.Thereisnostatisti- comparabletothoseinourstudy.Kocevskietal.(2012)analysed 4
callysignificantdifferencebetweenAGNhostgalaxiesandcontrol human classifications of host galaxies of X-ray-selected AGN at
galaxiesintheratesofveryhigh(A>0.1)asymmetrygalaxiesor z ≈ 2 with slightly higher luminosities than those studied in this
theratesofgalaxiesclassifiedasirregularbyhumanclassifiers.The paper.Cisternasetal.(2011)examinedthehostsofz∼1X-ray-
asymmetrydistributionsandhighasymmetryratesalsodonotshow selectedAGN.Allusedhumanclassifierstodeterminetheincidence
greaterdifferencesbetweenAGNhostsandcontrolasafunctionof of merger features and found no differences when compared to
eitherabsolutegalaxymagnitudeorAGNX-rayluminosity.There control. Grogin et al. (2005) and Boehm et al. (2013) both used
arenodifferencesbetweentheasymmetriesofX-rayobscuredand quantitative morphology measures on samples of X-ray-selected
unobscuredAGN.Theonlystatisticallysignificantdifferencefound AGN. Grogin et al. (2005) studied a sample of AGN at redshift
betweenAGNandcontrolgalaxiesisthatAGNhostgalaxieslo- (0.4<z<1.3)atthesamerestwavelengthasourstudyandBoehm
cated in a cluster environment show lower asymmetries as well etal.(2013)atz∼0.7andslightlybluerwavelengths.Bothstudies
as a higher skew, indicative of a stronger distribution tail at high foundnodifferencesbetweenAGNhostsandquiescentgalaxies.
asymmetries. Additionally, morphological inspections by human All these studies reach surface brightnesses where typical bright
classifiersshowthatAGNhostshavehigherratesofcompanions, mergerfeaturesshouldbevisible(Elmegreenetal.2007).Theyuse
althoughtheseshownosignsofinteraction.Thisdifferenceisdue similardepthsofimagingobservationsaswellasAGNluminosities
toAGNfoundintheclusterenvironment. asinthisstudyandallbutDunlopetal.(2003)useX-rayselection.
MNRAS439,3342–3356(2014)
CANDELS:hostgalaxiesofz ∼0.7AGN 3351
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Figure12. RatesofdisturbancesinAGNhostgalaxiescomparedtocontrol.Theleft-handpanelshowstheratesasafunctionofX-rayluminosity,right-hand y o
panelasafunctionofhostgalaxyabsolutemagnitude.Thetoppanelisfromquantitativeasymmetrymeasurements,thetwobottompanelrowsarefromhuman n M
classifiers,inparticular,weshowirregularity(allgalaxiesshowingsomeirregularfeatures)andmergers(galaxieseithershowingclearsignsofinteractionor a
havingclose-byneighbours).Overplottedare68.75percent(1σ)confidenceintervals. y 2
9
, 2
0
1
Therest-framewavelengthstudiedvariesslightlybetweenstudies. disruptionofsatellites.Thismakesinterpretationofsuchfindings 4
ThisimpliesthatourfindingsaretypicalofX-ray-selectedAGN. challenging.
Very deep imaging studies have revealed differences between OtherstudiesanalysedthehostgalaxiesofAGNwithoutcom-
AGN and control galaxies in some cases. Ramos Almeida et al. paringtocontrolsamples.Liu,Shen&Strauss(2012)andLetawe,
(2011) found that the host galaxies of radio-loud z < 0.7 AGN Letawe&Magain(2010)studiedthehostgalaxiesoflow-redshift
have disturbances with significantly higher surface brightnesses ( z <0.3) luminous quasars and found high incidences of merger
than found in quiescent galaxies. Ramos Almeida et al. (2011) signatures. Canalizo & Stockton (2001) studied a small sample
interpretthesesignaturesassignsofminormergersorfly-byinter- oflow-redshiftquasarswithULIRG-likeFIRSEDsandluminosi-
actionsinradiogalaxiesthataremorerecentintheAGNhoststhan ties and found high levels of disturbance in their host galaxies.
inthegeneralgalaxypopulation.Bennertetal.(2008)alsofound However, those sources were selected to be IR luminous. Their
weak signs of interaction, consistent with either major or minor host galaxy morphologies differ from the general quasar popula-
mergersabout1Gyrinthepastinasampleoflocalquasars.Koss tionthatshowslowlevelsofinteraction(Bahcalletal.1997).Ur-
etal.(2010)studiedlow-luminosityhard-X-ray-selectedAGNus- rutiaetal.(2008)studiedasampleofhighlyreddenedquasarsat
ingverydeepimagingdataandfoundstrongersignsofinteraction z∼1withstrongreddeningandlargeincidenceofoutflows.Nearly
in AGN compared to quiescent galaxies. However, weak merger 100percent of the red quasars show strong signs of recent inter-
featurescanbeduetomajororminormergersinthedistantpastor action.ThesequasarsareamongstthemostluminousAGNinthe
MNRAS439,3342–3356(2014)